The invention relates generally to solar cells for converting solar energy to electrical energy, and more particularly to improving efficiency of solar cells.
The demand for electrical energy continues to grow, yet the cost of producing such energy, especially by burning fossil fuels, seems to grow even faster. Generating electricity from solar energy using solar cells has for many years loomed on the horizon as a seemingly inexhaustible method for obtaining electricity, without attendant pollution. However the relatively low efficiency by which prior art solar cells convert solar energy to electricity has yet to be satisfactorily overcome. Solar-to-electrical energy conversion efficiency is on the order of about 15% for consumer grade solar cells, and is slightly higher for military grade solar cells used in outer space, which cells often employ a complex 3/5 compound substrate material to try to boost conversion efficiency.
Solar cells absorb photons from light energy, e.g., energy from the sun, and generate an electrical voltage and/or current in response. The incoming photon energy releases electron and hole pairs within a semiconductor substrate bulk that typically is silicon material. The solar cell structure includes one or more semiconductor p-n junctions whose internal electric fields help separate holes and electrons generated by the photon energy. The solar cell includes two terminals from which solar cell-generated voltage and/or current may be provided.
FIG. 1 depicts a typical prior art solar cell 10. Exemplary cell 10 is commonly fabricated on a semiconductor substrate (or bulk) 20 that is typically high resistivity long lifetime n-type silicon having a light receiving surface 30 and a second surface 40. Si is used more commonly than Ge in fabricating solar cells because the open circuit voltage (Voc) for Si is about 0.6 V whereas Voc for Ge is only about 0.2 V. Further, solar cells tends to get hot (even when used indoors), and Si can operate at higher temperatures than Ge.
In FIG. 1, a-plurality of n-type regions 50-N and p-type regions 50-P is formed adjacent second surface 40. Using convention fabrication techniques, the upper surface 40 of the structure will be masked (mask not shown) and implants of n-type dopants made into what will be regions 50-N, and implants of p-type dopant into what will be regions 50-P. Next, the dopants are diffused into the substrate material, typically by application of thermal energy. Given that substrate 20 is n-type material, p-n diode junctions are formed at the interface of p-type regions 50-N with the surrounding n-type substrate material 20. Although not required for solar conversion, cell 10 is shown with a cavity 60 formed in light-receiving surface 30. Cavity surface 70 may be formed with a conditioned surface that may include surface passivation, anti-reflection coating(s), and perhaps lens structures.
In practice, metal interconnections (not shown) will be made on surface 40, which interconnections block some of the incoming light. Accordingly it is preferred that cavity 60 be formed to allow light to penetrate surface 70 without blockage from metal interconnections on surface 40. Further, concentration of dopants is higher in regions near at surface 40 than in regions near surface 70. Regions near surface 40 will therefore characterized by shorter recombination lifetimes than regions at the junction depths where photon energy from light that enters the structure from the cavity direction generates electron-hole pairs.
In the configuration shown in FIG. 1, when cavity surface 70 is exposed to incoming light, the associated photon energy can release electron-hole pairs (denoted by xe2x80x9c-xe2x80x9d and xe2x80x9coxe2x80x9d symbols) within substrate 20. Some electron-hole pairs will recombine in the depletion region associated with the various p-n diodes. Other electron-hole pairs can diffuse into these depletion regions where they will recombine. A high percentage of electron-hole pairs will recombine in the substrate bulk without contributing to the generation of voltage or current signals.
The intrinsic electric field associated with the p-n regions (e.g., diode regions) produces a drift in the recombination, and produces an electrical current. This current is collected by metal or other conductive electrode structures (not shown) on surface 40 of solar cell 10, which electrodes are coupled to the p-junctions or to the n-junctions.
Conversion loss or inefficiency can result from electron-hole recombination that occurs before the hole-electron pairs can move to the junction depletion regions and contribute to the drift current. This loss can be especially troublesome at high energy, shorter wavelength regions and results in reduced conversion efficiency and decreased effective output voltage and/or current from the solar cell.
Conversion efficiency of solar cells is also affected by the wavelength of the incoming solar energy, and by the absorption coefficient of the materials comprising the solar cell. FIG. 2 depicts the absorption coefficient as a function of photon energy for Ge, Si, and GaAs. The data shown in FIG. 2 was obtained by Dash and Newman, et al. and the figure itself was originally published by John Wiley (c) 1981. Note that the absorption coefficient per centimeter versus photon energy varies from Ge to Si to GaAs.
In general, Ge absorption is about one to two orders of magnitude better than Si, which is strong at photon energy levels from about 1.1 eV to about 3.5 eV. Note that at low energy levels of about 0.6 eV (long wavelengths), Si has poor absorption but Ge exhibits good absorption. In essence, the bandgap for Ge and Si determine the cutoff of the longer wavelengths, with little or no absorption where the photon energy is less than the bandgap. Thus, long wavelengths resulting in energy less than about 1.1 eV will not be absorbed or captured by Si, while wavelengths that result in less than about 0.6 eV will not be absorbed by Ge. In this respect, Ge can capture wavelengths from 0.6 eV to 1.1 eV, wavelengths that Si cannot capture. The cutoff for high energy, short wavelengths is due to hole-electron pairs generated near the material surface. These hole-electron pairs are limited by surface recombination velocity and shorter lifetimes due to a higher dopant concentration. Photons end up recombining before they can reach the depletion regions. Such recombination occurs at wavelengths that are short at energies of about 3.0 eV to about 3.5 eV and above, for Si and for Ge, and likely for SiGe as well.
Thus, there is a need for a solar cell that exhibits improved adsorption characteristics and improved conversion efficiency. Preferably such solar cell structure should include materials whose band gap is similar to Ge but whose Voc is similar to Si. Preferably such materials should exhibit high temperature characteristics similar to Si rather than to Ge.
The present invention provides such a solar cell.
A highly efficient solar cell includes a preferably high resistivity long-lifetime n-type Si substrate having a light receiving first surface that preferably defines a cavity, and an opposite second surface upon which electrical contacts may be formed. Incoming photon energy enters the solar cell via the light receiving first surface in the cavity region and frees electron-hole pairs within the solar cell structure. The structure of the present invention improves collection of such free charges, which improves conversion efficiency of the overall solar cell structure.
Si material absorbs and can convert photon energy from about 1.1 eV to about 3.5 eV, but is less efficient at lower energy levels. By contrast, Ge material can absorb and convert photon energy below 1.1 eV down to about 0.6 eV. The present invention combines aspects of both materials to provide a tandem solar cell that can absorb and convert photon energy from about 0.6 eV to about 3.5 eV. The tandem solar cell exhibits the open circuit voltage Voc for Si of about 0.6 V, and further exhibits the large short circuit current associated with Si.
During fabrication, alternating n-type and p-type regions are defined and implanted adjacent the second surface without immediately being diffused. This process step is preferably immediately followed with formation of an epitaxial layer of Sixe2x80x94Ge, followed immediately by a growth of silicon epitaxy to cap the Sxe2x80x94Ge material. Advantageously the epitaxial cap can reduce stress generated by the physical mismatch between the small size of Si atoms and the larger size of germanium atoms. Further, the Sixe2x80x94Ge region and overlying Si cap region provides additional material that helps bury the Sixe2x80x94Ge junction region from the new surface formed by the Si epitaxial cap.
Heat introduced during growth of the epitaxial layers produces diffusion of dopants previously implanted in the substrate material. As a result of this heat regime, some dopant diffuses downward into the Si bulk toward the first surface, and some dopant diffuses upward into the Sixe2x80x94Ge epitaxial layer region and into the overlying Si epitaxial cap region. Advantageously, p-type and n-type diode regions formed in the Si substrate are not at the surface of the structure, and surface recombination of electron-hole pairs generated from incoming photon energy is reduced. Further, numerous depletion regions are defined in the Si bulk, in the Sixe2x80x94Ge epitaxial layer, and even within the overlying Si epitaxial cap layer.
Junctions in the Si substrate and the junctions in the Sixe2x80x94Ge epitaxial layer exhibit different energy bandgaps and different characteristics in response to the wavelength of photon energy entering the solar cell. A tandem solar cell according to the present invention combines in series the bandgap characteristics of Si and Sixe2x80x94Ge with incoming photon energy first being exposed to the higher Si bandgap and then to the lower Sixe2x80x94Ge bandgap. The result is good absorption and energy conversion from 0.6 eV through 3.5 eV. Overall conversion efficiency, is improved, and can be as high as about 50% to 60%.
Other features and advantages of the invention will appear from the following description in which the preferred embodiments have been set forth in detail in conjunction with the accompanying drawings.